US2561043A - Apparatus for grinding combustible materials - Google Patents

Description

July 17, 1951 J. w. AYERS 2,561,043

APPARATUS FOR GRINDING COHBUSTIBLE MATERIALS Filed July 12, 1946 JOSZPH 14/. AYl-RS BY I a fl ATTONEYS l atented July 17, 1951 APPARATUS FOR GRINDING COMBUSTIBLE MATERIALS Joseph W. Ayers, Easton, Pa., assignor to Agrashell, Inc., a corporation of Delaware Application July 12, 1946, Serial No. 683,226

Claims. (Cl. 241-31) This invention relates to an apparatus for grinding organic materials and more specifically to an apparatus including a grinding mill for the production of combustible materials in finely divided form in which all danger of explosion, burning, decomposition or deterioration of the product due to oxidation or other causes is avoided.

Highly efiicient grinding mills and processes have been developed for pulverizing inorganic materials but these cannot without modification be applied to wood, shells and other organic materials because the heat developed and the oxygen in the surrounding atmosphere tend to cause and have caused serious fires and explosions. Mills providing for cooling during the grinding of inorganic material by means of a large volume of air circulated through the mills cannot be used in the grinding of organic material because of the hazards arising from the presence of the oxygen.

In an effort to overcome these fire hazards, many safety devices and cut-out systems have been installed on grinding machinery to prevent fires or their spreading and to localize explosions. Grinding systems have been designed wherein the avoidance of oxygen in the mill is attempted by the introduction of inert gases such as carbon dioxide, nitrogen and steam. Since most ground organic materials however, have limited commercial value, the use of carbon dioxide, nitrogen, or other inert gas, even when the same is recirculated through the system has been found to be unsuitable or too expensive for industrial acceptance. To a limited extent wood flour has been ground on mill stones at temperatures below the boiling point of water where there is no fire hazard and also limited amounts of water and sometimes steam have been applied to the stones to cool the same. These mill stones and aids are employed where ample water power is available but they are not satisfactory for general use.

At the present time, various'organic materials are ground on corrugated and spiked differential rolls, in various attrition mills and in ball mills, but in spite of magnetic and other separating devices for removing tramp metal, of various safety devices and of various cutout systems, such mills are not 100% eflicient. In spite of all precautions taken, serious explosions have occurred on many occasions and because of inability to completely eliminate the explosion hazard, some manufacturers have been forced to discontinue operation of their plants.

Explosions, fires or oxygen deterioration of into the feeding mechanism or the mill. Whenv this increased amount of material reaches the grinding surfaces in the mill a substantial increase in energy is" exerted upon the material, and as a result, the temperature in the mill increases proportionately.

A broader object of'the present invention is to provide an apparatus or grinding system for rapidly grinding cbmbustible materials without any danger of fires and explosions or deterioration of the material being ground. A more immediate object is to provide an explosion proof apparatus for the rapid grinding of combustible materials which includes a means for preventing any substantial increase in temperature due to suddenly increased loads in the mill.

Generally considered, the invention may be described as an apparatus for the grinding of combustible materials comprising an enclosed grinding mill of a normally hot or rapid operating type having connected thereto sealed feeding and discharge means, a conduit having a regulating valve means therein for introducing water in a small amount or other cooling fluid into the mill, and automatic means responsive to changes in the load or amount of material fed into the mill operably connected to said valve. In the preferred embodiment of the invention, the automatic control means is only indirectly responsive to changes in the load but instead is directly responsive to the temperature in the mill effected by changes in the load. Hence a thermostat is mounted in the mill in such manner as to contact with the material being ground and such thermostat is operatively connected with the valve, suitably a needle valve, in the conduit through which the water is introduced into the mill.

The thermostat is preferably mounted in the discharge outlet from the mill or in the discharge mechanism. It cannot with complete success be mounted in the mill wall or other parts of the mill away from the ground material, for the temperature lag is too great to accomplish quick adjustment of the water flow. The regulation of the valve in the water feed conduit can be made responsive to the increase in amperage in the grinding motor circuit; but this method is not satisfactory or as simple as the thermostatic control hereinbefore described.

The water introduced into the mill through the conduit is a relatively small amount and does not change the character of the grinding operation from a dry to a wet process. The water evaporates andprovides steam within the mill. In order to maintain a controlled and limited amount of superatmospheric pressure in the mill, the mill is provided with a pressure release valve which preferably is mounted in an aspirating vent pipe having a suitable damper therein.

The water introduced into the mill removes or dissipates the excess heat developed therein through the latent heat of evaporation. Since water is continuously introduced into the mill at a predetermined and controlled rate in response to the temperature of the material being ground, increases in the rate of feed of the material to be ground to the mill cause only a very limited increase in temperature. In the preferred embodiment, in which the rate of fiow of water into the mill is controlled automatically by means or the thermostat mounted in such a manner that it is responsive to the temperature of the material being ground, the control instrument is set at a given temperature above the boiling point of water. Through this arrangement, mill temperatures may be held within F. of the setting of the instrument. During the grinding operation, effected at a temperature above2l2 F., steam is constantly generated in the mill thereby creating a positive pressure above atmospheric within the enclosed mill (1 to'3 pounds excess being a safe amount) thus excluding oxygen from the grinding operation. Under these conditions explosions and fires cannot take place and no deterioration due to oxidation is possible in the mill.

The present invention is not limited to any particular type of mill, for there may be used conventional dry grinding hammer mills, attrition mills, mill stones, disk mills, roller mills or disintegrators now used for or capable of grinding any of the many types of combustible materials contemplated by the present invention. The mill employed, however, must be totally enclosed in order that the superatmospheric pressure required may be developed and maintained. Sealed feed and discharge devices, such as rotary valves and interrupted or plug seal screw conveyors, must also be employed. For the release of any excess steam generated in the mill, the mill or conveying system may be provided with an aspirating pipe fitted with an adjustable damper so adjusted at all times as to keep the mill atmosphere under pressure.

The apparatus of the invention is designed primarily for the production of fine particle size powders and industrial fiours from various combustible organic materials and particularly for the manufacture of very fine particle sizes such as those which will pass a 100 mesh screen, a 200 mesh screen, a 325 mesh screen or still smaller sizes. It is not designed for the grinding of such materials to particles of coarse sizes in excess of 60 mesh, for ordinarily high temperatures or other deteriorative conditions are not encountered when coarse mesh material is produced. Only such apparatus is contemplated as is commonly referred to as hot-operating mills, i. e., mills.

in which the motivating energy and internal friction loss therein are high and in which a considerable amount of heat is generated in the mill which unless dissipated in some way will cause the material being ground to burn, explode, decompose, or otherwise be damaged by oxygen or other conditions.

The apparatus of the invention is suitable for the treatment of a great variety of natural vegetable materials such as wood for producing wood flour, nut shells for producing nut shell fiour and also pits, seeds, roots, tubers, stems, fibers, barks, grains and hulls for the production of fillers, extenders, adhesives, agricultural dustingpowders and the like. The apparatus is also suitable for the grinding of many synthetic combustible materials such as resins, organic pigments and other organic materials in general which do not appreciably soften at temperatures below 300 C.

In the grinding of natural vegetable materials by the apparatus of the present invention, the conversion of pentosans to furfural and organic acids such as acetic, formic, and gallic acids is avoided. Also the oxidation of the resin and wax content of such materials to undesirable end products is inhibited.

The apparatus of the present invention employs an attrition mill in preference to any other type. A suitable mill of this type is one equipped with 36 inch disks powered with horse power motors on the respective disks which run in opposite directions. The heat dissipation factor which the present invention must take into account can be appreciated from the knowledge that the horse power connected load on this mill converted to heat energy represents approximately 380,000 B. t. u. per hour. In the grinding of a material to a fine particle size it is reasonable to assume that at least 60% of the energy input is converted to heat by friction and with respect to the 36 inch attrition mill the heat developed would amount to about 230,000 B. t. -u. per hour. In the operation of the apparatus of the present invention, an amount of water is continuously introduced into the mill which dissipates sufiicient heat to maintain the temperature below that at which fires, explosions or product decomposition will result, which amount of water, of course, must not be suflicient to take the temperature of the mill below the required 212 F. for otherwise a superatmosphere of steam would not be maintained.

The present invention is exemplified by the accompanying drawing where a somewhat diagrammatic sectional view of a mill, sealing mechanisms and a control device are shown. With reference to the drawing, there is illustrated a conventional attrition mill in having opposing discs II and I2 which rotate in opposite directions. These discs have the grinding faces I 3 attached thereto in the usual way. Y

A hopper I for the material to be ground is connected to an inlet or feed conduit l5 having a screw conveyor therein, in turn joined to a con- 7 duit l6, having a rotary seal valve therein, and leading to the mill Ill. The mill I0 is provided with outlet conduits l1 and I8, the latter having an interrupted screw conveyor l9 therein which during operation creates a plug seal in the space 20. The conduit [8 is connected to the discharge outlet 2| for the ground material, from which the material is conveyed to a classifying system or to a packagingmachine (not shown). In the conveyor l8 there is a vent pipe 22 provided with an adjustable damper 23 adapted to maintain the the introduction of water.

required pressure in the mill and to relieve excess water vapor. In the outlet H, a thermostat 24 is mounted, the same being connected through control line 25 to a valve 26 in the conduit 21 for The thermostat and valve may be of any suitable construction and the former may be of the fluid type and the latter of the diaphragm operated type.

In a 36 inch attrition mill of this construction operated by two 75 horse poweer motors, 800 pounds per hour of a 325 mesh nut shell flour may be produced, the mill operating at a temperature of 275 F. The temperature is maintained substantially at this figure by introducing from .1 to .5 gallon of water per minute into the mill.

The amount of water introduced into the mill in any specific operation to obtain the required temperature is determined by the amount of water in the raw shell material fed to the mill, the

atmospheric conditions and radiation losses, the

clearance settings of the mill, the fineness of the finished product, the amount of recirculating load to the mill and the resistance of the raw material to grinding. The amount of moisture in the raw shell material, due to the nature of the material and atmospheric conditions, ordinarily will vary between 8 and 10% and is generally reduced during the grinding operation to 2 to 4% in the hot, dry ground product obtained. In view of this moisture variation, th automatic control of the .water supply added by means of a thermostat on the mill provides an extremely simple but positive metho of controlling the temperature while simultaneously effecting exclusion of oxygen.

Although the described apparatus is best and most easily operated with water in the liquid state introduced through the conduit 21 to reduce and control the temperature, it can be successfully operated with steam itself introduced at a temperature lower than that at which the mill is to operate and in a quantity sufiicient to effect the required cooling. The quantity of steam introduced is, of course, responsive to the thermostat 24 connected to the control valve 26. The apparatus of the invention not only has the advantage of effectinggrinding in a nonexplosive and non-oxidizing atmosphere but it also, by automatic control of the temperature in the mill, permits the mill to operate at its maximum capacity without danger of explosion.

It should be understood that the present invention is not limited to the specific details of construction herein disclosed except as set forth in the appended claims and that it extends to all'equivalents which will occur to those skilled in the art upon consideration of I claim: 1. In hot-operating, dry-grinding mills involv ing during use danger of fire, explosion or overheating of the material being ground due to frictional heat generated by the grinding opera tion, the combination of elements which comprises, an enclosed mill of said type having opposed grinding elements of a construction capable of grinding material in the dry state, said mill having connected thereto an inlet conduit conthe said claims. v

taining a sealed dry-material feeding means therein, an outlet conduit for the ground material, a sealed, air excluding, mechanically-operated, continuously operable dry-material discharge means in said outlet conduit, a conduit having a regulating valve therein connected to said mill for introducing a cooling fluid into the I mill, .and a thermostat responsive to the tem perature created by the grinding operation operably connected to said valve in such manner as to open and close the same respectively, whereby any temperature increase in the mill causes opening of the valve and an increase in the amount of cooling fluid fed into the mill, thus reducing the temperature and avoiding heat damage and danger of fire and explosion.

2. In hot-operating, dry-grinding mills involving during use danger of fire, explosion or overheating of the material being ground due to frictional heat generated by the grinding operation, the combination of elements which comprises, an enclosed mill of said type having opposed grinding elements of a construction capable of grinding material in the dry state, said mill having connected thereto an inlet conduit containing a sealed dry-material feeding means, an outlet conduit for the ground material, a sealed, air excluding, mechanically-operated, continuously operable dry-material discharge means in said outlet conduit, a conduit having a regulating valve therein connected to said mill for introducing a cooling fluid into the mill, a thermostat mounted substantially in the discharge end of the mill in such position as to contact with the freshly ground material, said thermostat being operably connected to said valve in such manner as to open and close the same respectively, whereby any increase in load and resultant temperature increase in the mill causes opening of the valve-and an increase in the amount of cooling fluid fed into the mill, thus reducing the temperature and avoiding heat.

damage and danger of fire and explosion.

3. In' hot-operating, dry-grinding attrition mills capable of producing powders finer than 60 mesh size, involving during use danger of fire, explosion or over-heating of the material being ground due to frictional heat generated by the grinding operation, the combination of elements which comprises, an enclosed mill of said type having opposed grinding elements of a construction capable of grinding material in the dry state, said mill having connected thereto an inlet conduit containing a sealed dryematerial feeding means, an outlet conduit for the ground material, a sealed, air excluding, mechanically-operated, continuously operable dry-material discharge means in said outlet conduit, a conduit having a regulating valve therein connected to said mill for introducing a cooling fluid into the mill, and a thermostat mounted substantially in the discharge end of the mill in such position as to contact with the freshly ground material, said thermostat being operably connected to said valve in such manner as to open same respectively whereby any increase in load and resultant temperature increase in the mill causes opening of the valve and an increase in the amount of cooling fluid fed into the mill, thus reducing the temperature and avoiding heat damage and danger of fire and explosion.

4. In hot-operating, dry-grinding attrition mills capable of producing powders finer than 60 mesh size, involving during use danger of fire, explosion or over-heating of the material being ground due to frictional heat generated by the grinding operation, the combination of elements which comprises, type having opposed grinding elements of a construction capable of grinding material in the dry state, said mill having connected thereto an inlet conduit containing a sealed dry-material feeding means, an outlet conduit for the ground material, a sealed, air excluding mechanically and close the an enclosed mill of said operated, continuously operable dry-material discharge means insaid outlet conduit, a. conduit having a regulating valve therein connected to said mill for introducing a cooling fluid into the mill, a thermostat mounted substantially in the outlet end of the mill at a point where it contacts with freshly ground combustible material, which thermostat is operably connected to said alve in such manner as to open and close the same respectively, whereby any increase in load and resultant temperature increase in the mill causes opening of the valve and an increase in the amount of cooling fluid fed into the mill, thus reducing the temperature and avoiding heat damage and danger of fire and explosion, and a steam pressure release valve connected to the mill for maintaining a limited superatmospheric pressure in the mill.

5. In hot-operating, dry-grinding attrition mills capable of producing powders finer than 60 mesh size, involving during use danger of fire, explosion or over-heating of the material being ground due to frictional heat generated by the grinding operation, the combination of elements which comprises, an enclosed mill of said type having oppositely rotatable opposed grinding elements of a construction capable of grinding material in the dry state, said mill having connected thereto an inlet conduit containing a screw conveyor leading from a hopper and containing a vertical section leading from the screw conveyor directly to the center of the grinding elements, a sealed star dry-material feeding means in said vertical section of the inlet conduit from which feeding means the material to be ground can freely fiow to the grinding elements, an outlet conduit for the ground material directly beneath said mill into which the ground material can freely fall, an air-excluding interrupted screw conveyor discharge means in said outlet conduit, a conduit having a regulating valve therein connected to said mill and terminating adjacent to a grinding element therein for introducing a cooling fluid into the mill, a thermostat mounted substantially in the outlet end of the mill at a point where it contacts with freshly ground combustible material, which thermostat is operably connected to said valve in such manner as to open and close the same respectively, whereby any increase in load and resultant temperature increase in the mill causes opening of the valve and an increase in the amount of cooling fiuid fed into the mill, thus reducing the temperature and avoiding heat damage and danger of fire and explosion, and a steam pressure release valve connected to the mill for maintaining a limited superatmospheric pressure in the mill.

JOSEPH W. AYERS.

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